The present invention relates generally to a method and apparatus for extracting carbon dioxide (CO2) from a stream or volume of gas, and sequestering said CO2 from the atmosphere or other gaseous environment. The invention particularly relates to a method and apparatus that utilize carbonate and water to sequester said CO2 as bicarbonate.
Description of Related Art
A variety of chemical means exist or have been proposed which consume CO2 contained in emissions from fossil fuel combustion or other gas streams, thus reducing the potential atmospheric CO2 burden (reviews by: H. Herzog and E. Drake, “Carbon Dioxide Recovery and Disposal From Large Energy Systems', Annual Reviews of Energy and Environment Vol. 21, p 145-166, 1996; X. Xiaoding and J. A. Moulijn, “Mitigation of CO2 by Chemical Reactions and Promising Products”, Energy and Fuels, Vol. 10, p 305-325, 1996). Among these chemical approaches, the exposure and reaction of such waste CO2 to certain naturally occurring or artificially formed calcium-, magnesium-, sodium-, and/or silica-rich minerals has been explored as reviewed below. The reaction of certain carbonate and silicate minerals with CO2 is a well-known “rock weathering” phenomenon that plays a major role in controlling atmospheric CO2 on geologic time scales (R. A. Berner, A. C. Lasaga, and R. M. Garrels, “The Carbonate-Silicate Geochemical Cycle and its Effect on Atmospheric Carbon Dioxide Over the Last 100 Million Years”, American Journal of Science, Vol. 283, p 42-50, 1983). Over the very long term such process are expected to eventually consume most of the CO2 emitted by man's activities. The problem is that such natural processes occur on the order of >1,000 year time scales and thus will have little immediate impact on the rapidly increasing CO2 emissions and atmospheric CO2 burden in the coming centuries. Nevertheless, several researchers have proposed that certain weathering reactions be used to sequester CO2, in particular those reactions which lead to CO2 sequestration or storage in the form of solid carbonates.
For example, fixation and storage of CO2 by artificial weathering of waste concrete in combination with coccolithophorid algae cultures was reported by H. Takano and T. Matsunaga, “CO2 Fixation by Artificial Weathering of Waste Concrete and Coccolithophorid Algae Cultures”, Energy Conversion Management, Vol. 36, No. 6-9, p 697-700, 1995. It was shown that CO2 can be sequestered into biologically produced carbonate and biomass. Various mechanisms of rock weathering to fix CO2 was discussed by T. Kojima, “Evaluation Strategies for Chemical and Biological Fixation/Utilization Processes of Carbon Dioxide”, Energy Conversion Management, Vol. 36, No. 6-9, p 881-884, 1995. Studies of CO2 fixation by silicate rock weathering were reported by T. Kojima, A. Nagamine, N. Ueno and S. Uemiya, “Absorption and Fixation of Carbon Dioxide by Rock Weathering”, Energy Conversion Management, Vol. 38, Suppl., p S461-S466, 1997. Sequestering of CO2 as carbonate by reaction with minerals rich in calcium and magnesium oxides was reported by K. S. Lackner, C. H. Wendt, D. P. Butt, E. L. Joyce, D. H. Sharp, “Carbon Disposal in Carbonate Minerals”, Energy, Vol. 20, No. 11, p 1153-1170, 1995. Reacting flue gas CO2 with water and soil to ultimately precipitate and sequester the CO2 as carbonate was explored by T. Chohji, M. Tabata, and E. Hirai, “CO2 Recovery From Flue Gas by an Ecotechnological (Environmentally Friendly) System”, Energy, Vol. 22 No. 2/3, p 151-159, 1997. A study by H. Kheshgi (“Sequestering Atmospheric Carbon Dioxide by Increasing Ocean Alkalinity”, Energy, Vol. 20, No. 9, p 912-922, 1995) looked at the option of adding calcium oxide to the ocean as a means of increasing the CO2 absorption capacity of the ocean. The preceding approaches often require elevated temperatures or pressures, significant energy, land, or other resource inputs, and/or have negative environmental impacts. The cost of implementing these technologies is therefore often prohibitive.
As reviewed by H. Herzog and E. Drake, (Annual Reviews, loc. cit.) several chemical means exist for separating and concentrating CO2 from gas streams. U.S. Pat. No. 4,376,101 (Sartori et al) discloses the removal of CO2 from a gaseous stream via use of an aqueous solution containing an alkali metal salt or hydroxide and an activator or promoter system comprising an amine compound. While such processes remove or separate CO2 from a waste stream, they offer no downstream method of ultimately sequestering the CO2 from the atmosphere. They also often require elevated temperatures or pressures, exotic chemicals, and/or significant inputs of energy or resources.
Gas/water/calcium carbonate (limestone) reactors have been used in desulfurization of power plants exhaust as reviewed by H. N. Soud and M. Takeshita, “FGD Handbook, IEA Coal Research, London, 438p., 1994. Such reactors differ from the present invention in three important aspects: 1) The volume of SO2 in the gas streams to which desulfurization is applied is vastly smaller than the CO2 content in the same gas stream; 2) The hydration step in carbonate desulfurization involves combining SO2 with H2O to form the strong acid H2SO3. In contrast, the hydration of CO2 envisioned here forms carbonic acid H2CO3, a weak acid which has a slower reaction rate with carbonate than does H2SO3. 3) The reaction of H2SO3 with carbonate (e.g., CaCO3) and oxygen forms a solid, CaSO4, and a gas, CO2, whereas the H2CO3 with carbonate reaction forms cations and bicarbonate in solution, does not require supplemental oxygen, produces little or no solid waste, and consumes rather than generates gaseous CO2.
U.S. Pat. No. 5,100,633 (Morrison) describes a process for scrubbing acid-forming gases which include SO2 and CO2 from an exhaust gas stream through reactions with alkaline solutions formed from the waste ash from biomass burning. The resulting alkali metal salts are then precipitated or dewatered forming solid, possibly useful waste products. This process does not provide a system for net CO2 sequestration, however, considering that the molar ratio of carbon to alkali metals or to alkaline earth metals in the end products is many times lower than that ratio in the original biomass burned to form the alkaline ash. That is, only a very small fractional equivalent of the CO2 released in biomass combustion can be sequestered by this process, and therefore when initial ash and CO2 formation are considered the overall process is a net source rather than a net sink for CO2.
The chemical reactions involving CO2 gas, water, and carbonate minerals (principally calcium carbonate) have been extensively studied as reviewed by J. W. Morse and F. T. Mackenzie (“Geochemistry of Sedimentary Carbonates”, Cambridge, Amsterdam, 707p., 1990) and by T. Arakaki and A. Mucci (“A Continuous and Mechanistic Representation of Calcite Reaction-Controlled Kinetics in Dilute Solutions at 25° C. and 1 Atm Total Pressure”, Aquatic Geochemistry, Vol. 1, p 105-130, 1995). However, the context of these studies has been to describe the dissolution or precipitation of solid carbonate under various conditions, not the consumption and sequestration of CO2.
Due to its relative simplicity, low-cost, and low environmental impact, it is believed that the invention herein disclosed offers distinct advantages over other methods for the combined process of extracting CO2 from waste gas streams and sequestering this CO2 from the atmosphere.